1. Field of the Invention
The present invention relates to methods of making microelectronic spring contact arrays, such as contact arrays for connecting to semiconductor devices (singulated or unsingulated), for purposes of testing or assembly.
2. Description of Related Art
Microelectronic spring contact arrays such as used for contacting C4 or flat pad terminals of semiconductor devices have previously been made in various different ways. Some older techniques involve the assembly of fine, stiff components, such as tungsten wires, onto a base, such as a probe card. Techniques using tungsten wire and like components are generally limited to contact arrays with relatively few contacts, because of practical difficulties associated with achieving and maintaining a precise contact tip alignment across the array.
A more recent method, involving forming composite spring contacts on a substrate using a relatively soft, fine wire that is coated with a layer of stiffer material, is capable of producing higher contact densities than the older tungsten wire techniques. The composite contacts may be formed directly on a contactor base or tile, or may be formed on a sacrificial substrate and transferred to a contactor base later. In the case of composite contacts that are transferred, loose contacts may be assembled to the contactor base or tile using a “pick-and-place” technique (i.e., by individual handling), or by gang-transferring to a contactor substrate. In a gang-transfer technique, the composite spring contacts are first formed tips-down on the sacrificial substrate. Then, while still attached to the sacrificial substrate, the contacts are first attached to a contactor substrate at their bases, and then, the sacrificial substrate is removed.
Composite contacts are subject to some limitations. The shape of composite contacts is somewhat limited by the wire shaping process. Also, the soft wire core of each composite contact generally requires individual shaping before being coated with stiffener. This may slow down the process of making an array, particularly for arrays that include many thousands of such contacts.
In yet another method, microelectronic spring contacts are formed on a contactor base using lithographic techniques that are similar to techniques for making semiconductor devices. The contactor base is coated with one or more sacrificial layers, and the sacrificial layers are patterned to define a contoured surface extending up through the sacrificial layers from the contactor base of each desired contact. A suitable spring contact material is then deposited on each contoured surface, and the sacrificial layers are removed to reveal freestanding spring contacts. Lithographic techniques have the advantage of enabling more varied shapes to be used for spring contacts, as well as eliminating the need for individual handling of the spring contacts. However, relatively complex lithographic processes may be needed to make spring contacts of certain shapes, and to achieve certain configurations of spring contacts on contactors, such as overlapping contacts.
In some prior art methods, the composite and lithographic methods described above are combined to form a spring contact that includes both a composite portion, and a lithographically formed portion. Combination methods combine certain advantages of composite and lithographic methods, while still being subject to the disadvantages of both.
It is desirable, therefore, to provide a method of making microelectronic spring contact arrays that overcomes the limitations of prior art methods.